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Mushroom Networks Blog

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What is MOS (Mean Opinion Score) for VOIP?

MOS, Mean Opinion Score is a measure of voice quality, and is a quality measure that has been used in telephony for decades as a way to assess the human users opinion of call quality. The test is used widely in VoIP networks to ensure quality voice transmission, test for quality issues, and provides a metric by which to measure voice degradation and performance. With the increased popularity of VoIP phone services, MOS scoring is essential to ensuring client satisfaction for continued network growth. Here is a Wikipedia link for the definition of MOS.

How is MOS, Mean Opinion Score determined?

By its very name “Opinion”, the MOS, Mean Opinion Score was a subjective measurement used to test the listener’s perception of the voice quality, and clarity of the communication. The test was performed in a ‘quiet room’ meeting specific size and noise requirements, in which listeners would receive and score calls on the quality as they perceived it. VoIP MOS score measurements, on the other hand, is usually more objective, the score providing a measure of the network over which it is carried.

MOS testing for VoIP phone networks is defined in the ITU-T PESQ P.862 standard, and is algorithm based rather than relying on live subjects opinions. The test is modeled on the subjective tests that were commonly used previously, assessing the voice quality of human beings using true voice samples as test signals. For accurate results, it’s vital that modern telecom equipment be loaded with speech-like signals they are optimized for to avoid unpredictable, or unreliable results.

How is the MOS, Mean Opinion Score measured?

Because call quality is highly subjective, there are several ways in which the score can be assessed, including algorithms that are used to predict MOS scores and are a common option for VoIP networks. Regardless, human involvement is by far the most effective, but not always the most practical way to score MOS within a network of decent size. The final MOS is an average across the participants, resulting in a score between 0-5, with 5 being an excellent quality call, and 0 being indecipherable.

On the other hand, in many cases modern tests rely heavily on algorithms that focus on modem response time, codec speed and complex tests to predict how the voice quality would be perceived by the human ear. Real voice signals are used to test clarity, delay, packet loss, jitter and determine a probable MOS score. Yes, this is an estimate to the human based MOS score, but it is more practical and scalable.

Once the data has been gathered, calculations to factor in the R factors are done. These are factors that can degrade call quality that aren’t due to network error. These include propagation delay (the time required for a digital signal to travel across the network), packetization delay (the time required to digitize the signal), and jitter buffer (a delay used to collect voice packets in order to process them at more regular intervals).

What factors does the MOS, Mean Opinion Score, take into account?

The score reports on three different factors:

Listening Quality

Transmission Quality

Conversational Quality

Testing may focus on all three factors simultaneously, or focus on a single aspect at a time. During testing a group of participants monitor calls for a controlled sample, rating each one individually. Measurements are collected for after-testing the latency of the connection or the one-way delay to determine ways to improve the network. In automated tests an algorithm is used to rate voice signals in an attempt to predict how a human ear would rate the quality, and predict a MOS score.

What affects your VoIP MOS test score?

It’s important to understand that the MOS score is measured on a relative scale, built on a variety of factors that can affect voice quality. Because of this, there are many factors that can affect the MOS score on VoIP system that would not be a factor on a regular phone line. These include:

Hardware

Bandwidth

Jitter

Latency

Packet Loss

Codec Version

Codec version is one factor that can have a serious impact on the MOS score. A non-compressed codec provides the best voice quality, with the lack of compression and decompression making the signal less susceptible to audio quality loss. Depending on the compression ratio, compression can degrade voice quality significantly.

Although compression/decompression systems can be configured to conserve bandwidth, there is still a significant trade-off with voice quality. The best codecs for VoIP to ensure high MOS, Mean Opinion Scores are those that provide maximum bandwidth conservation, and minimal voice quality degradation. Also recent codecs that support HD-Voice (High Definition Voice) can tilt the balance in the other direction where higher quality is obtained in lieu of bandwidth efficiency.

MOS, Mean Opinion Score is an essential resource for VoIP providers and clients to ensure high quality service, maximum voice quality and provide techs with the needed information to enhance voice quality on VoIP calls. The score is a measure of voice clarity, and can be either subjective or objective depending on the method used.

Cahit Akin, CEO, Mushroom Networks, Inc.

Mushroom Networks is the provider of the VOIP Armor device that enables self-healing WAN networks for VOIP / SIP traffic by routing around network problems such as latency, jitter and packet loss.